We investigate the effects of superhydrophobic surfaces (SHS) carrying streamwise grooves on the ﬂow dynamics and the resultant drag reduction in a fully developed turbulent channel ﬂow. The SHS is modelled as a ﬂat boundary with alternating no-slip and free-slip conditions, and a series of direct numerical simulations is performed with systematically changing the spanwise periodicity of the streamwise grooves. In all computations, a constant pressure gradient condition is employed, so that the drag reduction effect is manifested by an increase of the bulk mean velocity. To capture the ﬂow properties that are induced by the non-homogeneous boundary conditions the instantaneous turbulent ﬂow is decomposed into the spatial-mean, coherent and random components. It is observed that the alternating no-slip and free-slip boundary conditions lead to the generation of Prandtl’s second kind of secondary ﬂow characterized by coherent streamwise vortices. A mathematical relationship between the bulk mean velocity and different dynamical contributions, i.e. the effective slip length and additional turbulent losses over slip surfaces, reveals th ... mehrat the increase of the bulk mean velocity is mainly governed by the effective slip length. For a small spanwise periodicity of the streamwise grooves, the effective slip length in a turbulent ﬂow agrees well with the analytical solution for laminar ﬂows. Once the spanwise width of the free-slip area becomes larger than approximately 20 wall units, however, the effective slip length is signiﬁcantly reduced from the laminar value due to the mixing caused by the underlying turbulence and secondary ﬂow. Based on these results, we develop a simple model that allows estimating the gain due to a SHS in turbulent ﬂows at practically high Reynolds numbers.